Volume analysis of supercooled water under high pressure
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.426
Volume analysis of supercooled water under high pressure Solomon F. Duki1 and Mesfin Tsige2 1 National Center for Biotechnology Information, National Library of Medicine and National Institute of Health, Bethesda MD, 20894 USA
2
Department of Polymer Science, The University of Akron, Akron OH, 44325 USA
ABSTRACT
Motivated by an experimental finding on the density of supercooled water at high pressure [O. Mishima, J. Chem. Phys. 133, 144503 (2010)] we performed atomistic molecular dynamics simulations study of bulk water in the isothermal-isobaric ensemble. Cooling and heating cycles at different isobars and isothermal compression at different temperatures are performed on the water sample with pressures that range from 0 to 1.0 GPa. The cooling simulations are done at temperatures that range from 40 K to 380 K using two different cooling rates, 10 K/ns and 10 K/5 ns. For the heating simulations we used the slowest heating rate (10 K/5 ns) by applying the same range of isobars. Our analysis of the variation of the volume of the bulk water sample with temperature at different pressures from both isobaric cooling/heating and isothermal compression cycles indicates a concave-downward curvature at high pressures that is consistent with the experiment for emulsified water. In particular, a strong concave down curvature is observed between the temperatures 180 K and 220 K. Below the glass transition temperature, which is around 180 K at 1GPa, the volume turns to concave upward curvature. No crystallization of the supercooled liquid state was observed below 180 K even after running the system for an additional microsecond.
I-INTRODUCTION Water is a very important substance that exists naturally in its different phases in a wide range of temperature and pressure. This made it the subject of intense study in many interdisciplinary research areas that range from biological systems to large scale industrial application materials. One of the many questions that has been investigated over the years by both theorists and experimentalists is the understanding of the completely different behavior of water observed at low temperature compared to other liquids under the same set of conditions [1-4]. In particular, the structural change in supercooled water under high pressure has been examined from two different point of
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views that debate whether the liquid-liquid transition at low temperature is continuous or discontinuous. The discontinuity hypothesis asserts the existence of a liquid-liquid critical point (LLCP)[5] where transition between two liquids is discontinuous at the critical point. On the other hand, the singularity-free theory (SF) hypothesized that the liquid-liquid transition is continuous and singularity-
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